The electromagnetic spectrum is a limited and valuable resource allocated in the United States by the federal government, specifically the Federal Communications Commission (FCC). The FCC determines which types of applications are permitted to use which parts of the electromagnetic spectrum. Two radio frequency bands have been allocated by the FCC for use by airborne telecommunication systems. Communications with airborne telephones on commercial aircraft has been allotted bands from 849 to 851 megahertz (MHz) for uplink communications, i.e. transmissions to the airborne telephones, and from 894 to 896 MHz for downlink communication, i.e., transmissions from airborne telephones. Each band has 2 megahertz MHz) bandwidth, and the two bands are separated by 45 MHz. Both the uplink and downlink bandwidths are divided into 10 subbands, each 200 Kilohertz (KHz) wide. The subbands are further divided into 29 traffic service channels (a type of communication channel) and six pilot channels each. Thus a total of 290 traffic service channels are available for communication with airborne telephones. Each traffic service channel has a 6 KHz bandwidth in both the uplink and downlink frequency allotments.
As shown in FIG. 1 the electromagnetic broadcast frequency spectrum 10 allotted for communications with airborne telephones has a low band 12 and high band 14. Each band 12,14 has been divided into 10 subbands 16,18 of 200 KHz each, numbered from 10 down to 1. Each subband 16,18 has been further divided into a set of 6 numbered control channels (pilot channels) 20 and 29 traffic service channels 22. In accordance with the FCC Memorandum of Opinion and Order, each of the six control channels 20 has been given a bandwidth of 3.2 KHz, and each of the 29 traffic service channels 22, a bandwidth of 6 KHz. Guard bands of 2.5 KHz 24, 2.3 KHz 26 and 1.5 KHz 28 separate traffic service channels 22 from pilot channels 20 and from traffic service channels in different subbands. These channel assignments allow up to six service providers to offer nationwide airborne radiotelephone services simultaneously. Each will be assigned one of the numbered pilot channels. The pilot channel assigned to a specific service provider will be the same in each subband in each cell covered by a radio base station. All service providers will have equal access to the set of traffic service channels used in each cell. No service provider "owns": a traffic service channel, but each "owns" one control channel in each subband.
Finally, it is known to divide each traffic service channel into 2 user service channels. Each user service channel carries the communications between a phone on an airplane and another phone coupled to a radio base station. That radio base station must be serving a geographic area in close enough proximity to the aircraft to allow communication with the airplane.
The United States is blanketed with dozens of radio base stations. A radio base station is the suite of ground equipment required to process air-to-ground and ground-to-air calls. The ground stations are located throughout the US as well as Canada and Mexico. Typically, one radio base station is separated from another by 300 to 500 miles. Each radio base station is assigned a block of frequencies or subband(s) on which calls are processed. Subbands are assigned such that the same subband is not reused within 550 miles from the radio base station it is assigned to. This arrangement avoids co-channel interference, i.e., the same channel in use in overlapping cells.
The limited bandwidth allotted to communication with airborne telephones in combination with the number of available radio base stations serves to constrain the possible number of simultaneous calls, thus limiting the market for airborne telephonic communications. Therefore it is desirable to provide a system capable of utilizing the available spectrum with as high efficiency as possible while providing excellent quality communications to airborne customers.
When an aircraft radio unit on an aircraft acquires a traffic service channel both user service channels may or may not be utilized for a period of time, but typically one user service channel becomes unused before the other. This is because each user service channel is being used independently, i.e., any given call utilizing a user service channel is usually unrelated to a call utilizing the other user service channel on the same traffic service channel. In that case the aircraft will only be using one user service channel per traffic service channel, even though two user service channels are available per traffic service channel. This often happens on multiple traffic service channels resulting in multiple traffic service channels only being partially utilized. Note that present aircraft radio units have only two traffic service channels each. Unfortunately, partial utilization of multiple traffic service channels by one aircraft will preclude other aircraft from acquiring those traffic service channels or using the unused user service channel. Recall that a total of 290 traffic service channels is all that is presently available for airborne telephones. In present airborne telephone systems the described management of Traffic service channels can result in callers on other aircraft being precluded from making calls while unused user service channels exist but are unavailable. Therefore, it would be desirable for traffic service channels to be used more efficiently to minimize the number of partially utilized traffic service channels in order to increase the number of potential simultaneous calls.
Other problems arise with a mobile communications system, especially one which is deployed on commercial jet aircraft. For example, a conversation between a passenger on such a plane and someone on the ground or in another plane may continue long enough for the plane to fly from one cell into another. Note that in this case a cell is defined as the area wherein a radio base station provides a signal above a threshold necessary to provide quality communications and that cell areas may overlap. When this happens the call is eventually terminated as an aircraft flies out of an acceptable communications range. Presently, the only way to continue the conversation is for one of the parties to the conversation to redial the other party. It would be desirable for the mobile communications system to retain the connection between callers even though at least one of the callers is moving from one cell to the next. Furthermore, it would be desirable to retain the connection between callers with a minimum amount of interference when a caller crosses from one zone to the next.
As the caller begins leaving a particular cell zone the strength of the signal from the radio base station begins to diminish. As the signal continues to diminish communication becomes difficult, then impossible with existing equipment. Another related difficulty is the degradation of signal quality for reasons other than leaving a particular cell zone. For example, interference can cause noise on the channel, mailing communication difficult. It would be desirable to minimize problems with a noisy or weak signal strength channel being used in a mobile communication system. Furthermore, it would be desirable to be able to minimize the above problems with present aircraft radio units having a variety of different traffic service channel usage scenarios.